#ifdef LOCAL
#define _GLIBCXX_DEBUG
#else
#pragma GCC optimize("O3,Ofast,unroll-loops")
#endif
#include <iostream>
#include <fstream>
#include <sstream>
#include <iomanip>
#include <cstdio>
#include <vector>
#include <string>
#include <stack>
#include <queue>
#include <deque>
#include <bitset>
#include <algorithm>
#include <cmath>
#include <set>
#include <unordered_set>
#include <map>
#include <unordered_map>
#include <chrono>
#include <random>
#include <complex>
#include <numeric>
#include <assert.h>
#include <functional>
#include <climits>
#include <cstring>
#include <iterator>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;
using ll = long long;
using ld = long double;
using ull = unsigned long long;
using ushort = unsigned short;
#define int ll
#ifndef LOCAL
#define endl "\n"
#endif
#define f first
#define s second
#define vec vector
#define pii pair<int, int>
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define pb push_back
#define mp make_pair
template<typename T1, typename T2>
istream& operator>> (istream &in, pair<T1, T2> &p) { in >> p.f >> p.s; return in; }
template<typename T1, typename T2>
ostream& operator<< (ostream &out, pair<T1, T2> p) { out << "(" << p.f << " " << p.s << ")"; return out; }
#define printv(v) cerr << #v << " "; for (auto &x : v) { cerr << x << " "; } cerr << endl;
#define printvv(v) cerr << #v << " "; for (auto &x : v) { for (auto &y : x) { cerr << y << " "; } cerr << endl; }
#define debug(x) cerr << #x << " " << x << endl;
template<typename T>
istream& operator>> (istream &in, vector<T> &v) {
for (auto &x : v) {
in >> x;
}
return in;
}
template<typename T>
ostream& operator<< (ostream &out, vector<T> v) {
for (auto &x : v) {
out << x << " ";
}
return out;
}
template<typename T>
void operator-=(vector<T> &v, int delta) {
for (auto &x : v) {
x -= delta;
}
}
template<typename T>
void operator+=(vector<T> &v, int delta) {
for (auto &x : v) {
x += delta;
}
}
mt19937 rnd(chrono::steady_clock::now().time_since_epoch().count());
typedef tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update> ordered_set;
signed Main();
void Solve();
void PreCalc();
static void run_with_stack_size(signed (*func)(), size_t stsize) {
char *stack, *send;
stack = (char *)malloc(stsize);
send = stack+stsize-16;
send = (char *)((uintptr_t)send/16*16);
asm volatile(
"mov %%rsp, (%0)\n"
"mov %0, %%rsp\n"
:
: "r" (send));
func();
asm volatile(
"mov (%0), %%rsp\n"
:
: "r" (send));
free(stack);
}
signed main() {
#ifdef LOCAL
freopen("in.txt", "r", stdin);
int tStart = clock();
run_with_stack_size(Main, 1024*1024*1024);
cout << endl << "execution time : " << (clock() - tStart) / 1e3 << endl;
#else
ios::sync_with_stdio(false);
cin.tie(0);
Main();
#endif
}
signed Main() {
PreCalc();
int t = 1;
// cin >> t;
int test = 0;
while (t--) {
++test;
#ifdef LOCAL
cout << "--------- TEST #" << test << " ----------" << endl;
#endif
Solve();
}
return 0;
}
/* This code is praying to GOD for protecting from bugs and "other things" */
void PreCalc() {
}
void Solve() {
int n, m;
cin >> n >> m;
vector<int> b(m), p(m);
for (int i = 0; i < m; ++i) {
cin >> b[i] >> p[i];
}
priority_queue<pii> q;
vector<bitset<30001>> has(n);
vector<vector<int>> moves(n);
for (int i = 0; i < m; ++i) {
moves[b[i]].pb(p[i]);
has[b[i]][p[i]] = 1;
}
for (int i = 0; i < n; ++i) {
sort(all(moves[i]));
}
vector<int> d(n, 1e18);
vector<int> marked(n, 0);
q.push(mp(0, b[0]));
d[b[0]] = 0;
while (!q.empty()) {
int v = q.top().s;
q.pop();
if (marked[v]) {
continue;
}
marked[v] = 1;
int lst = -1;
for (auto &x : moves[v]) {
if (lst == x) {
continue;
}
lst = x;
int k = 1;
while (v + k * x < n) {
if (d[v + k * x] > d[v] + k) {
d[v + k * x] = d[v] + k;
q.push(mp(-d[v + k * x], v + k * x));
}
if (has[v + k * x][x]) {
break;
}
++k;
}
k = 1;
while (v - k * x >= 0) {
if (d[v - k * x] > d[v] + k) {
d[v - k * x] = d[v] + k;
q.push(mp(-d[v - k * x], v - k * x));
}
if (has[v - k * x][x]) {
break;
}
++k;
}
}
}
if (d[b[1]] == 1e18) {
cout << -1 << endl;
} else {
cout << d[b[1]] << endl;
}
}
